The present invention relates to electric resistance heating elements and, more particularly, to an insulating support for the helical wire coil used in such heating elements.
Electric heating elements utilizing helical wire heating coils are old and well known in the art. A helical wire heating coil is typically mounted on a supporting structure and strung between a number of ceramic insulating elements which provide direct support for the coil and space and isolate the coil from the supporting structure which is generally some type of metal framework. It is important that the insulating element hold the coil against both lateral displacement out of the supporting member and movement in the direction of the axis of the coil. Thus, it is common in prior art ceramic insulating supports to capture one or more turns of the helical coil to hold the same against lateral displacement and axial movement.
One common prior art support is typified by the constructions shown in U.S. Pat. Nos. Des. Nos. 261,260 and 262,285 and U.S. Pat. Nos. 4,363,959 and 4,692,599. In each of these Patents, a ceramic insulating support for the helical coil of a heating element includes a generally thin flat body with two or more hook-like notches on one or both ends. A few turns or convolutions of the coil are separated slightly and retained in the hook-like notches by the inherent resilience of the coil. The axis of the coil extends generally parallel to the thin flat body of the insulator with adjacent turns of the coil held in oppositely facing notches. To attach the coil to these supports, the coil must be stretched axially and/or twisted rather severely from its axial direction, resulting in the possibility of stretching the wire beyond its yield point and causing a permanent deformation in the coil.
Another somewhat similar insulating support is shown in U.S. Pat. No. 4,250,399. This insulator also has a relatively thin flat ceramic body with a single coil supporting notch centered in one edge. The notch extends generally perpendicular to the flat body and supports the coil with its axis also perpendicular to the body (or generally at 90 degrees from the coil axis in the constructions previously described). The edge of the insulator body on both sides of the notch is provided with downwardly opening lips which engage the coil turns on each face of the body to prevent the coil from being withdrawn after attachment. In order to attach the coil to the insulator body, however, it must be turned so that the coil axis is at 90 degrees to its final position in order to insert one turn of the coil into the slot. This assembly procedure also has the potential for causing permanent distortion of the coil and, where multiple supports are used to mount a long heating coil as is customary, the procedure for mounting the coil on the entire supporting framework can be tedious and time consuming, as well.
The insulating supports of the prior art typically include a body with a single notch in one edge or an elongate body having oppositely opening notches in opposite edges. The supporting structure for mounting the coil-carrying supports may comprise a formed sheet metal structure or circular cross section wire rods. Generally, a single long helical coil extends in multiple parallel passes, generally four, over the supporting framework and the two ends of the coil are attached to a conventional electric terminal for connection to a source of electric current.
It would be most desirable to have an insulating support for a helical wire heating coil to which the coil may be attached without bending or twisting the coil off of its longitudinal axis, but which allows the coil to be attached easily and without permanent distortion, and which retains the coil against either axial or lateral movement.